As a recording media for reading an information by an optical method, various types of optical recording media (hereinafter referred to as “optical disks”) have been developed, each of which has an information recording layer formed on a light-incident side plane and a cover layer made of a transparent resin for covering the information recording layer. As these optical disks, e.g. CD optical disks or DVD optical disks have been known. Meanwhile, optical head devices for writing and/or reading (hereinafter referred to as “writing/reading”) an information to/from such DVD optical disks, each of which has e.g. a laser diode as a light source for emitting light of 660 nm wavelength band and an objective lens having a NA (numerical aperture) of from 0.6 to 0.65.
Heretofore, as an optical disk for DVD, an optical disk (hereinafter referred to as “single layer DVD optical disk”) having a single information recording layer with a cover thickness (thickness of cover layer) of 0.6 mm, and an optical disk (hereinafter referred to as “double layer DVD optical disk”) having double information recording layers (read only type or readable and writable type) and the like have been developed. In such a double layer DVD optical disk, the distance between the information recording layers is 55±15 μm, and the information recording layers in the light-incident side are formed at positions where the cover thickness is is from 0.56 mm to 0.63 mm.
Accordingly, in a case of carrying out writing/reading to/from a double layer DVD optical disk by using an optical head device employing an objective lens optimally designed to make an aberration zero for a single layer DVD optical disk of cover thickness 0.6 mm, spherical aberration is generated according to the difference of cover thickness, which deteriorates convergence of incident light to the information recording layer. Particularly, for a write only type double layer DVD optical disk, deterioration of convergence causes lowering of convergent power density at a time of writing and causes writing error, such being problematic.
Further, in recent years, in order to improve recording density of an optical disk, an optical disk (hereinafter referred to as “single layer BD optical disk”) having a cover thickness of 100 μm has been developed. Meanwhile, for an optical head device to be used for writing/reading such a single layer BD optical disk, e.g. a laser diode of blue light of wavelength 405 nm band as a light source and an objective lens having a NA of 0.85 are employed. In a case of writing/reading a single layer BD optical disk by using such an optical head device, if the cover thickness varies ±5 μm in a plane of single layer BD optical disk, a large spherical aberration of about 50 mλ in terms of RMS (Root Mean Square) wavefront aberration occurs, which deteriorates convergence of light incident into an information recording layer, such being problematic.
Further, a double layer optical disk of write-only type having cover thicknesses of 100 μm and 75 μm (hereinafter it is also referred to as “double layer BD optical disk”) has also been developed, but in this double layer BD optical disk, a large spherical aberration corresponding to the difference between cover thicknesses causes writing error, such being problematic.
To cope with these problems, heretofore, as means for correcting a spherical aberration generated due to e.g. the difference of cover thicknesses of optical disks, methods using movable lens group or a liquid crystal lens as described in the following applications have been known.
(I) For example, JP-A-2003-115127 proposes, in order to correct spherical aberration by using a movable lens group, an optical head device 100 for writing/reading an optical disk D as shown in FIG. 29. This optical head device 100 has a light source 110, various types of optical system 120, a photo-receiving element 130, a control circuit 140, a modulation/demodulation circuit 150, and further, first and second movable lens groups 160 and 170. Further, in the first movable lens group 160, a concave lens 161, a convex lens 162 and an actuator 163 are provided. Accordingly, by moving the is convex lens 162 fixed on an actuator 163 in an optical axis direction, a focal length movable lens function is exhibited, in which the power of the movable lens group 160 continuously changes between positive (convex lens) and negative (concave lens). By disposing the movable lens group 160 in an optical path to the optical disk D, it is possible to adjust convergent point of incident light on information recording layers of optical disks D having different cover thicknesses, and thus, it becomes possible to correct a spherical aberration containing a power component.
(II) Further, JP-A-5-205282 proposes, in order to correct spherical aberration generated due to the difference of cover thickness between an optical disk for DVD and an optical disk for CD, an optical head device employing a liquid crystal lens 200 as shown in FIG. 30. The liquid crystal lens 200 comprises a substrate 230 having a flat surface on which a transparent electrode 210 and an alignment film 220 are formed, a substrate 260 having a curved surface symmetric about an axis and has a surface shape S(r) represented by formula (1) being a power series of radius r on which a transparent electrode 240 and an alignment film 250 are formed, and a nematic liquid crystal 270 sandwiched between these substrates.S(r)=a1r2+a2r4+a3r6  (1)                wherein a1, a2, a3; constantr2=x2+y2         
In the liquid crystal lens 200, when a voltage is applied between the transparent electrodes 210 and 240, alignment of molecules in the liquid crystal 270 changes and the refractive index changes. As a result, according to the refractive index difference between the substrate 260 and the liquid crystal 270, transmission wavefront of light incident into the liquid crystal lens 200 changes.
(III) Further, JP-A-9-230300 proposes, in order to exhibit substantial lens function of changing a power component corresponding to change of convergent point of incident light without increasing the thickness of a liquid crystal layer, an optical modulation element as a liquid crystal lens. Further, the reference proposes an optical head device employing such an optical modulation element to correct a spherical aberration generated due to the difference of cover thickness between an optical disk for DVD and an optical disk for CD. FIG. 31 shows a side view of the optical modulation element 300. The optical modulation element 300 comprises substantially parallel two transparent substrates 310 and 320 and a liquid crystal 330 sandwiched between them, and a liquid crystal side surface of the transparent substrate 310 has a concentric blaze shape 340, and on liquid crystal side surfaces of the two transparent substrates, electrodes 350 and alignment films 360 are formed. In the optical modulation element 300, alignment direction of the liquid crystal 330 is substantially in parallel with the transparent substrates at a time of no electric field application, and the alignment direction is substantially perpendicular to the transparent substrates at a time of electric field application.
(IV) Further, JP-A-9-189892 proposes, in order to exhibit substantial lens function of changing a power component corresponding to change of convergent point of incident light without increasing the thickness of a liquid crystal layer, a liquid crystal diffraction lens 400 as shown in FIG. 32.
In the liquid crystal diffraction lens 400, a substrate 410 has one surface on which a predetermined saw-tooth shaped relief is formed, a transparent electrode 420 is formed on the surface, and a liquid crystal layer 440 is sandwiched between the transparent electrode 420 and an opposing electrode 430. When a voltage is applied between these electrodes 420 and 430, substantial refractive index of the liquid crystal layer 440 for extraordinarily polarized light changes from extraordinary refractive index ne to ordinary refractive index no. Here, substantial refractive index means an average refractive index in the thickness direction of the liquid crystal layer.
Provided that the refractive index of the substrate 410 having a saw-tooth-shaped relief structure is n1 and wavelength of incident light is λ, by forming the saw-tooth-shaped relief so that a depth d of a groove of the saw-tooth-shaped relief satisfies the relation of the following formula:d=λ/(ne−n1),maximum diffraction efficiency is obtained at a wavelength λ at a time of no voltage application and a diffraction lens is constituted. Further, even if wavelength λ of incident light changes, it is possible to adjust applied voltage so that maximum diffraction is obtained at the wavelength λ.
In the liquid crystal diffraction lens 400 having such a construction, since it is sufficient to fill the liquid crystal layer 440 with a liquid crystal so as to fill grooves of the saw-tooth-shaped relief, it is possible to reduce the thickness of the liquid crystal layer 440 as compared with the type of liquid crystal layer for correcting a spherical aberration containing a power component by constituting the above-mentioned liquid crystal lens 200 shown in FIG. 30.